Two-Dimensional Boron Monolayer Sheets

Boron, a nearest-neighbor of carbon, is possibly the second element that can possess freestanding flat monolayer structures, evidenced by recent successful synthesis of single-walled and multiwalled boron nanotubes (MWBNTs). From an extensive structural search using the first-principles particle-swarm optimization (PSO) global algorithm, two boron monolayers (alpha(1) and beta(1)-sheet) are predicted to be the most stable alpha- and beta-types of boron sheets, respectively. Both boron sheets possess greater cohesive energies than the state-of-the-art two-dimensional boron structures (by more than 60 meV/atom based on density functional theory calculation using PBEO hybrid functional), that is, the alpha-sheet previously predicted by Tang and Ismail-Beigi and the g(1/8)- and g(2/15)-sheets (both belonging to the beta-type) recently reported by Yakobson and co-workers. Moreover, the PBEO calculation predicts that the alpha-sheet is a semiconductor, while the alpha(1)-, beta(1)-, g(1/8)-, and g(2/15)-sheets are all metals. When two a, monolayers are stacked on top each other, the bilayer alpha(1)-sheet remains flat with an optimal interlayer distance of similar to 3.62 angstrom, which is close to the measured interlayer distance (similar to 3.2 angstrom) in MWBNTs.